Cooking surface with optimized distance between induction coil and cooking vessel

ABSTRACT

A cooking surface has a thickness between 0.05 and 3.0 mm to optimize between highest efficiency, lowest magnetic stray field, and desired surface material. In the thickness range of 0.05 to 0.2 mm, a top layer may consist of plasma sprayed hard ceramic such as alumina which can be impregnated with poly-tetra-fluor-ethylene. In the thickness range of 0.2 to 2.0 mm, the top layer may consist of glass fibre reinforced material which can be impregnated with poly-tetra-fluor-ethylene and can be of a hardened quality. In the thickness range of 1.00 to 3.00 mm, the top layer may consist of a vitroceramic or glass material.

BACKGROUND OF THE INVENTION

The invention relates to a cooking surface for an induction heatingcooker comprising a coil structure below the surface.

Flat and heat tolerant upper surfaces for cookers are well known, andtheir use in domestic kitchens is predominantly due to the ease withwhich one may move cooking utensils from one to the other cooking zone,the ease of cleaning, and the quick thermal response. However, most ofthese are made selfsupporting of a glass or vitroceramic material whichhave to be quite thick in order to withstand the dropping of cookingutensils. The differences in coefficient of expansion between the topplate of said glass or vitroceramic material and the supporting metalframe make the manufacture complex and the cleaning of the edges of thetop plate is rather more difficult due to the need to allow expansionwith respect to the surrounding frame.

The fairly thick glass or vitroceramic top plates which have becomeknown in connection with radiative heating zones have been carried overinto the area of induction heating, mainly because of thewell-established manner of manufacture, provision with patterns andsigns and because the material combines electrical insulation withstrength. The hampering in energy transfer due to the spacing betweenthe transmitting coil and the cooking utensil has been accepted in viewof the other advantages.

In W094/05137 a cooking plate concept is described which utilises aninduction heating arrangement comprising a ring-shaped magnetic corestructure made of magnetically conductive concrete. This structure isembedded in further concrete material, and the top surface may be eithercast upon the fairly solid base or be made of possibly porous alumina(aluminium oxide ceramic) which demonstrates a high thermal insulationand abrasion resistance. This latter material would be applied by plasmaspraying. It is hence non-selfsupporting. However, this kind of topsurface material, although thermally excellent, has shown certaindisadvantages in practice. The inherent porosity makes the surfacedifficult to clean, although very strong detergents may be used, due tothe chemical stability of alumina. The casting in place of a top layerof heat resistant concrete is efficient in the manufacturing process,but the finishing of such a surface and the sealing of the pores topermit cleaning are difficult tasks and the result may be destroyed bydetergents. Hitherto it has been considered that a top layer should beas thin as possible for efficient power transport, in other words, theair gap should be minimized. Hence all endeavours in this field haveworked towards this goal. The structure described above provides thispossibility in a most efficient manner, but at the cost of a verymassive construction.

BRIEF SUMMARY OF THE INVENTION

There is hence a need for an improved surface which does not display theabove-identified disadvantages, yet still provide the functionality of aconstruction of the type described in said PCT publication.

This is obtained by applying the realization that the heating of the topsurface is due to two contributions: losses in the induction energytransmitter which is embedded in the base at some distance from theupper surface of the heat insulating layer and conduction of heat fromthe heated cooking vessel via the heat insulating layer to the inductionenergy transmitter. Said heating actually increases the losses of theinduction energy transmitter. In fact, it has surprisingly turned outthat for each combination of permeability in the bottom of the cookingvessel and the range of power handled by the cooking zone there is arange of thicknesses of the top layer which provides an optimum ofefficiency in the transfer of power from the cooking zone to the cookingutensil. It has turned out that such thicknesses all fall in the sameabsolute range which is in all cases below the 3 mm which is in generalindustrial use today. The improved structure is characteristic in thatthe thickness of the cooking surface is chosen between 0.05 mm and 3 mm,preferably between 0.2 and 2.5 mm. The preferential distance alsoaffords a good performance as regards magnetic field leakage. Thespecific thickness to be used in any particular combination of coilstructures, operational frequency and cooking vessel range may bedetermined by the skilled person without undue experimentation.

In an embodiment of the invention the surface is particular in that itextends unbroken over the totality of the coil structures at least tothe edge area of the cooking area. This will give certain advantages asconcerns cleaning by washing and may be supplemented by raised edgesformed in the same material.

A further embodiment extends directly from the basic idea of theinvention in that the top layer has a limited area, though larger than acoil structure, and may be removeable but flush with the surroundingsurface when fitted. This makes it possible to fit surfaces above thecoil structure which may be changed when eroded, permanently soiled, orperhaps not fitting with the internal decoration of the kitchen area,all in all making this highly technological product much more adaptableto living conditions than previously known cookers.

Further embodiments of the invention adapts the material of the topsurface to the thickness determined for the particular construction,viz. for the thickness range 0.05 to 0.2 mm the top layer consists ofplasma sprayed hard ceramic such as alumina, possibly impregnated withpoly-tetra-fluor-ethylene, and for the thickness range 0.2 to 2 mm thetop layer consists of a glass fibre reinforced material, possiblyimpregnated with poly-tetra-fluor-ethylene, possibly of a hardenedquality. For the thickness range 1.00 to 3.00 mm the top layer consistsof a vitroceramic or glass material. These materials each combine goodwearing qualities with good cleaning properties, however according tothe invention they are so thin that they would not be able to supportthemselves over large areas, let alone carry cooking vessels.

A further material which may advantageously be used in embodiments ofthe invention is an organically modified ceramic material.

In order to permit the user to determine which coil structure (whichcooking zone) has most recently been active, a further advantageousembodiment incorporates reversible heat-modifiable pigments into thesurface. This is now feasible and with a wide range of fairlylow-temperature pigments because the invention provides for a highefficiency and hence low losses and reduced heating of the surfaceitself.

The specified thickness and the consequent reduction of heating lossesaccording to the invention makes it possible to construct an inductionheating cooker with several cooking zones which is totallyself-supporting, i.e. without brackets between the edge frame membersand yet lightweight. This is obtained by a construction which isparticular in that one or several transmitter coil structure orstructures is/are embedded in a polymer structure which is composed witha compartmentalised underside and a plane topside incorporating shallowwells for exchangeable top layers above each transmitter coil. A similarembodiment uses a light alloy in stead of a polymer, with the specificadvantage that stray fields from the coil structures are shielded orshort-circuited.

It has furthermore been realized that it is possible to obtain athickness in the upper part of the range and yet retain the materialsfor the lower part of the thickness range with the advantages particularto those materials in that the surface consists of a protective layerand a number of projections directed upwards from said layer, the heightof the projections defining the thickness of the cooking surface. Thereis no limitation to the area and number of these projections, however incase only very few are used, they must not be so sharp as to scratch thecooking utensils. Obviously a minimum number of three supportingprojections are needed for a pot. The particular advantage of thisembodiment must be seen in the layer of air which creates heatinsulation between the bottom of the cooking vessel and the coilstructure embedded in the cooker.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The invention will be described in greater detail in the following withreference to the drawings, in which

FIG. 1 shows the general relationship between efficiency-and thicknessof the top layer,

FIG. 2 shows the general relationship between the magnetic B fieldsurrounding a pot on an induction heating cooker in dependence of thethickness of the top layer,

FIG. 3 shows structures for top surfaces according to the invention, and

FIG. 4 shows a structure in which an air space supplies heat insulationbetween the bottom of a pot and the induction heating coil structure.

DETAILED DESCRIPTION OF THE INVENTION

The basis for the invention resides in an observation that the practicalefficiency (determined as output power versus input electrical power) ofan induction heating cooker 1 is dependent on the distance between thetop of the coil structure 2 including the core parts and has a maximum,the sharpness of which is dependent on the actual construction but withthe general feature that thicknesses in the range 0.05 to 3 mm cause ahigher efficiency than thicknesses outside this range. A general curvedisplaying this relationship is shown on FIG. 1. Similarly it has beenobserved as shown on FIG. 2 that the induction field B surrounding a pot3 on an induction heating coil structure increases with distance atrelatively small distances. Hence there may surprisingly be found is adistance which is a suitable compromise between the highest efficiency,the lowest magnetic stray field, and the desired surface material.

In FIG. 3 is shown embodiments of the invention corresponding to therelevant thickness ranges. It is advantageous to let the surface layer,or top layer 4, extend outside the position of the induction coilstructures, and hence the embodiments are generally shown to besupported by the "supporting structure" 5.

In FIG. 4 is shown an embodiment which displays high strength andcleanability combined with the creation of an air gap which wouldnormally only be obtained in an extremely porous structure. Thesupporting structure carries an upper surface made of alumina or similarcomposition which displays rounded projections, the number anddistribution of which is dependent on the desired appearance of the top.The area of the individual projections may be selected according to e.g.the types of pot and may be ribs, possibly in a radial distribution.

We claim:
 1. A cooking surface for an induction heating cooker used toheat a cooking vessel, said cooking surface comprising:a supportstructure layer having a supporting surface, the support structure layercontaining a coil structure, the coil structure having a top surfacelocated below the supporting surface, a non metallic and nonself-supporting top layer having an upper surface defining a supportplane for the cooking vessel, the top layer disposed on the supportingsurface at least in an area above the coil structure, the supportingsurface providing subjacent support for the top layer and the cookingvessel in at least the area above the coil structure, wherein a distancebetween said support plane and said top surface of said coil structureis between 0.05 mm and 3 mm.
 2. A cooking surface according to claim 1,wherein there are a plurality of said coil structures to form aplurality of cooking zones and said top layer extends unbroken over eachof the coil structures.
 3. A cooking surface according to claim 1,wherein said top layer has an area smaller than said support structure,though larger than said top surface of said coil structure.
 4. A cookingsurface according to claim 2, wherein said distance is in the range of0.05 to 0.2 mm and the top layer consists of plasma sprayed hardceramic.
 5. A cooking surface according to claim 2 or 3, wherein saiddistance is in the range of 0.2 to 2 mm and the top layer consists of aglass fibre reinforced material.
 6. A cooking surface according to claim2 or 3, wherein said distance is in the range of 1.00 to 3.00 mm and thetop layer consists of a material selected from a vitroceramic materialand a glass material.
 7. A cooking surface according to claim 1, whereinsaid top layer consists of an organically modified ceramic material. 8.A cooking surface according to claim 1, wherein said top layerincorporates reversible heat-modifiable pigments.
 9. A cooking surfaceaccording to claim 1 or 2, wherein the supporting surface of saidsupport structure forms a shallow well above said coil structure andsaid top layer is located within said well and is removable from saidwell.
 10. A cooking surface according to any one of claims 1-3, whereinsaid support structure is a polymer structure, said coil structure isembedded in said polymer structure, said polymer structure has acompartmentalized underside and a plane topside forming a shallow wellabove said coil structure, and said top layer is located within saidwell and is removable from said well.
 11. A cooking surface according toany one of claims 1-3, wherein said support structure is aself-supporting light alloy structure and said coil structure is fittedinto said self-supporting light alloy structure, said light alloystructure has a plane topside forming a shallow well above said coilstructure, and said top layer is located within said well and isremovable from said well.
 12. A cooking surface according to any one ofclaims 1-3 and 7, wherein said top layer consists of a protective layerand a number of projections directed upwards from said protective layer,a top of the projections defining said support plane.
 13. A cookingsurface according to claim 1, wherein said distance is in the range of0.2 and 2.5 mm.
 14. A cooking surface according to claim 3, wherein saidtop layer is removable from said support structure and is flush withsurrounding portions of an upper surface of said support structure. 15.A cooking surface according to claim 4, wherein said sprayed hardceramic is selected from the group of alumina and alumina impregnatedwith poly-tetra-fluor-ethylene.
 16. A cooking surface according to claim5, wherein said glass fibre reinforced material is selected from thegroup of material impregnated with poly-tetra-fluor-ethylene andmaterial of a hardened quality.